Interactions of bioactive trace metals in shipboard Southern Ocean incubation experiments

In the Southern Ocean, it is well-known that iron (Fe) limits phytoplankton growth. Yet, other trace metals can also affect phytoplankton physiology. This study investigated feedbacks between phytoplankton growth and dissolved Fe, manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) concentrations in Southern Ocean shipboard incubations. Three experiments were conducted in September-October 2016 near the West Antarctic Peninsula: Incubations 1 and 3 offshore in the Antarctic Circumpolar Current, and Incubation 2 inshore in Bransfield Strait. Additions of Fe and/or vitamin B-12 to inshore and offshore waters were employed and allowed assessment of metal (M) uptake relative to soluble reactive phosphorus (P) across a wide range of initial conditions. Offshore, treatments of >1 nmol L-1 added Fe were Fe-replete, whereas inshore waters were already Fe-replete. Results suggest Mn was a secondary limiting nutrient inshore and offshore. No Fe-vitamin B-12 colimitation was observed. Overall, M:P uptake in the incubations was closely related to initial dissolved M:P for Fe, Mn, Co, Ni, and Cd, and for Cu inshore. Final concentrations of Fe and Zn were similar across light treatments of the experiments despite very different phytoplankton responses, and we observed evidence for Co/Cd/Zn substitution and for recycling of biogenic metals as inventories plateaued. In dark bottles, the absence of Mn oxidation may have allowed more efficient recycling of Fe and other trace metals. Our results provide insight into factors governing trace metal uptake, with implications for phytoplankton community composition locally and preformed micronutrient bioavailability in Southern Ocean water masses.

Automated summary

This study examined the relationship between phytoplankton growth and dissolved concentrations of iron, manganese, cobalt, nickel, copper, zinc, and cadmium in the Southern Ocean. The results showed that manganese was a secondary limiting nutrient both onshore and offshore, and there was no co-limitation of iron and vitamin B-12. The uptake of metals relative to soluble reactive phosphorus was closely related to initial dissolved metal to phosphorus ratio.